Proton-conducting membrane and use thereof

ABSTRACT

The present invention relates to a novel proton-conducting polymer membrane based on polyazole block polymers which, owing to their outstanding chemical and thermal properties, can be used widely and are suitable in particular as polymer electrolyte membrane (PEM) for producing membrane electrode units or so-called PEM fuel cells.

The present invention relates to a novel proton-conducting polymermembrane based on polyazole block polymers which, owing to theiroutstanding chemical and thermal properties, can be used widely and aresuitable in particular as polymer electrolyte membrane (PEM) inso-called PEM fuel cells.

Polyazoles, for example polybenzimidazoles (®Celazole) have been knownfor sometime. Such polybenzimidazoles (PBIs) are prepared typically byreacting 3,3′,4,4′-tetraaminobiphenyl with isophthalic acid ordiphenylisophthalic acid or their esters thereof in the melt. Theprepolymer formed solidifies in the reactor and is subsequentlycomminuted mechanically. Subsequently, the pulverulent prepolymer isfinally polymerized in a solid-phase polymerization at temperatures ofup to 400° C. and the desired polybenzimidazoles are obtained.

To prepare polymer films, the PBI, in a further step, is dissolved inpolar, aprotic solvents, for example by dimethylacetamide (DMAc) and afilm is obtained by means of classical processes.

Proton-conducting, i.e. acid-doped, polyazole membranes for use in PEMfuel cells are already known. The basic polyazole films are doped withconcentrated phosphoric acid or sulfuric acid and then act as protonconductors and separators in so-called polymer electrolyte membrane fuelcells (PEM fuel cells).

As a result of the outstanding properties of the polyazole polymers,such polymer electrolyte membranes, processed to give membrane-electrodeunits (MEE), can be used in fuel cells at long-term operatingtemperatures above 100° C., in particular above 120° C. This highlong-term operating temperature allows it to increase the activity ofthe noble metal-based catalysts present in the membrane-electrode unit(MEE). Especially in the case of use of so-called reformats made fromhydrocarbons, the reformer gas comprises significant amounts of carbonmonoxide which typically have to be removed by a complicated gas workupor gas purification. The possibility of increasing the operatingtemperature allows distinctly higher concentrations of CO impurities tobe tolerated on a long-term bases.

Use of polymer electrolyte membranes based on polyazole polymers firstlyallows complicated gas workup or gas purification to be partly dispensedwith and secondly allows the catalyst loading in the membrane-electrodeunit to be reduced. Both are unavoidable prerequisites for large-scaleuse of PEM fuel cells, since the costs for a PEM fuel cell system areotherwise too high.

The acid-doped polyazole-based polymer membranes known to date alreadyexhibit a favorable property profile. However, owing to the applicationsdesired for PEM fuel cells, especially in the automobile sector anddecentralized power and heat generation (stationary sector), they are inneed of improvement overall. Furthermore, the polymer membranes known todate have a high contact of dimethylacetamide (DMAc) which cannot fullybe removed by means of known drying methods. The German patentapplication No. 10109829.4 describes a polymer membrane based onpolyazoles in which the DMAc contamination has been eliminated. Althoughsuch polymer membranes exhibit improved mechanical properties, specificconductivities do not exceed 0.1 S/cm (at 140° C.).

The German patent application No.10117687.2 describes a novel polymermembrane based on polyazoles which is obtained starting from themonomers by polymerizing in polyphosphoric acid. In PEM fuel cells,especially in high-temperature PEM fuel cells, this membrane exhibitsoutstanding performance. However, it has been found that these membranesare still in need of improvement with regard to their mechanical stressin order also to ensure use under extreme conditions. Especially in theautomobile sector, a PEM fuel cell has to be able to be started up againwithout any problems even after being at rest at extremely low externaltemperatures. Condensed moisture can, especially at temperatures belowthe freezing point, result in considerable mechanical stress acting onthe membrane. In addition to these requirements, a higher mechanicaldurability of the membrane is also advantageous in the production of themembrane-electrode. For instance, considerable forces act on themembrane in the lamination, so that good stretchability and resiliencecan be advantageous.

It is an object of the present invention to provide acid-containingpolymer membranes based on polyazoles, which firstly have theperformance advantages of the polymer membrane based on polyazoles andsecondly have increased specific conductivity, especially at operatingtemperatures above 100° C., and additionally do not need moistening ofthe fuel gas.

We have now found that a proton-conducting membrane based on polyazoleblock polymers can be obtained when the parent monomers are suspended ordissolved in polyphosphoric acid and first polymerized up to a certaindegree, and these are then mixed and polymerized to block polymers.

The doped polymer membranes exhibit very good proton conductivity withsimultaneously high elongation at break.

The present invention provides a proton-conducting polymer membranebased on polyazoles, obtainable by a process comprising the steps of

-   A) mixing one or more aromatic tetraamino compounds having a high    phosphoric acid affinity or low phosphoric acid affinity with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer which have a high    phosphoric acid affinity or low phosphoric acid affinity, or mixing    one or more aromatic and/or heteroaromatic diaminocarboxylic acids    having a high phosphoric acid affinity in polyphosphoric acid to    form a solution and/or dispersion-   B) heating the mixture from step A), preferably under inert gas, and    polymerizing until an intrinsic viscosity of up to 1.5 dl/g,    preferably from 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g,    is obtained to form a polymer whose phosphoric acid affinity is    greater than the phosphoric acid affinity of the polymer formed in    step D),-   C) mixing one or more aromatic tetraamino compounds having a high    phosphoric acid affinity or low phosphoric acid affinity with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer which have a high    phosphoric acid affinity or low phosphoric acid affinity, in    polyphosphoric acid to form a solution and/or dispersion-   D) heating the mixture from step C), preferably under inert gas, and    polymerizing until an intrinsic viscosity of up to 1.5 dl/g,    preferably from 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g,    is obtained to form a polymer whose phosphoric acid affinity is less    than the phosphoric acid affinity of the polymer formed in step B),-   E) combining the polymer from step B) and the polymer from step D),    the phosphoric acid affinity of the polymer from step B) being    greater than the phosphoric acid affinity of the polymer from step    D),-   F) applying a layer using the mixture according to step E) on a    carrier or on an electrode,-   G) heating the sheetlike structure/layer obtainable according to    step F), preferably under inert gas, until an intrinsic viscosity of    more than 1.5 dl/g, preferably of more than 1.8 dl/g, in particular    of more than 1.9 dl/g, is attained to form a polyazole block    polymer,-   H) treating the membrane formed in step G) (until it is    self-supporting).

The aromatic and heteroaromatic tetraamino compounds used in accordancewith the invention and having a high phosphoric acid affinity arepreferably 2,3,5,6-tetraaminopyridine,3,3′,4,4′-tetraaminodiphenylsulfone, 3,3′,4,4′-tetraaminodiphenyl etherand salts thereof, especially the mono-, di-, tri- andtetrahydrochloride derivatives thereof.

The aromatic and heteroaromatic tetraamino compounds used in accordancewith the invention and having a low phosphoric acid affinity arepreferably 3,3′,4,4′-tetraaminobiphenyl, 1 ,2,4,5-tetraaminobenzene,3,3′,4, 4′-tetraaminobenzophenone, 3,3′,4,4′-tetraaminodiphenylmethaneand 3,3′,4,4′-tetraaminodiphenyldimethyl-methane and salts thereof,especially the mono-, di-, tri- and tetrahydrochloride derivativesthereof.

The aromatic carboxylic acids used in accordance with the invention aredicarboxylic acids and tricarboxylic acids and tetracarboxylic acids orthe esters thereof, especially the C1 -C20-alkyl esters or C5-C12-arylesters thereof, or the anhydrides thereof or the acid chlorides thereof.

The aromatic carboxylic acids used in accordance with the invention andhaving a high phosphoric acid affinity are preferablypyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazi nedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid,2-hydroxyterephthalic acid, 5-aminoisophthalic acid,5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic acid,2,5-dihydroxterephthalic acid, 2,6-dihydroxyisophthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,8-dihydroxynaphthalene-3,6-dicarboxylic acid anddiphenylsulfone-4,4′-dicarboxylic acid.

The aromatic carboxylic acids used in accordance with the invention andhaving a low phosphoric acid affinity are preferably isophthalic acid,terephthalic acid, phthalic acid, 3-fluorophthalic acid,5-fluoroisophthalic acid, 2-fluoroterephthalic acid, tetrafluorophthalicacid, tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, diphenyl ether 4,4′-dicarboxylic acid,benzophenone-4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid and 4-carboxycinnamic acid.

The diaminocarboxylic acids used in accordance with the invention andhaving a high phosphoric acid affinity are preferably diaminobenzoicacid and the mono and dihydrochloride derivatives thereof, and also1,2-diamino-3′-carboxy acid 4,4′-diphenyl ether.

The aromatic tri-, tetracarboxylic acids or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or the acid anhydrides thereof or the acidchlorides thereof are preferably 1,3,5-benzenetricarboxylic acid(trimesic acid); 1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl )iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or the acid anhydrides thereof or the acidchlorides thereof are preferably 3,5,3′, 5′-biphenyltetracarboxylicacid; benzene-1,2,4,5-tetracarboxylic acid; benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids used in accordance with theinvention are heteroaromatic dicarboxylic acids and tricarboxylic acidsand tetracarboxylic acids or the esters thereof or the anhydridesthereof. Heteroaromatic carboxylic acids are understood to mean aromaticsystems which contain at least one nitrogen, oxygen, sulfur orphosphorus atom in the aromatic. They are preferablypyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,and also the C1-C20-alkyl esters or C5-C12-aryl esters thereof, or theacid anhydrides thereof or the acid chlorides thereof.

The content of tricarboxylic acid or tetracarboxylic acids (based ondicarboxylic acid used) is between 0 and 30 mol%, preferably 0.1 and 20mol%, in particular 0.5 and 10 mol%.

In step A), preference is also given to using mixtures of at least 2different aromatic carboxylic acids, the ratio of the monomers beingbetween 1:99 and 99:1, preferably from 1:50 to 50:1. It is thus possibleto use carboxylic acids having a high phosphoric acid affinity andcarboxylic acids having a low phosphoric acid affinity equally, althoughthe selection of the carboxylic acids and the mixing ratio are selectedso as to result, in the subsequent polymerization (step B), in a polymerwhose phosphoric acid affinity is above that of the polymer formed instep D).

In step A), preference is also given to using mixtures of at least 2different aromatic tetraamino compounds, in which case the ratio of themonomers is between 1:99 and 99:1, preferably from 1:50 to 50:1. It isthus possible to use tetraamino compounds having a high phosphoric acidaffinity and tetraamino compounds having a low phosphoric acid affinityequally, although the selection of the tetraamino compounds and themixing ratio are selected so as to result, in the subsequentpolymerization (step B), in a polymer whose phosphoric acid affinity isabove that of the polymer formed in step D).

It has been found that the total content of monomers having a lowphosphoric acid affinity based on all monomers used in step A) can betolerated up to 40% by weight, preferably of up to 25% by weight, inparticular from 0.1 to 25% by weight.

In step C), preference is also given to using mixtures of at least 2different aromatic carboxylic acids, in which case the ratio of themonomers is between 1:99 and 99:1, preferably from 1:50 to 50:1. It isthus possible to use carboxylic acids having a high phosphoric acidaffinity and carboxylic acids having a low phosphoric acid affinityequally, although the selection of the carboxylic acids and the mixingratio is selected so as to result, in the subsequent polymerization(step D, in a polymer whose phosphoric acid affinity is lower than thatof the polymer formed in step B).

In step C), preference is also given to using mixtures of at least 2different aromatic tetraamino compounds, in which case the ratio of themonomers is between 1:99 and 99:1, preferably from 1:50 to 50:1. It isthus possible to use tetraamino compounds having a high phosphoric acidaffinity and tetraamino compounds having a low phosphoric acid affinityequally, although the selection of the tetraamino compounds and themixing ratio is selected so as to result, in the subsequentpolymerization (step D), in a polymer whose phosphoric acid affinity islower than that of the polymer formed in step B).

It has been found that the total content of monomers having a highphosphoric acid affinity based on all monomers used in step C) can betolerated up to 40% by weight, preferably of up to 25% by weight, inparticular from 0.1 to 25% by weight.

In step E), the polymers obtained in steps B) and D) are mixed. Themixing ratio of the polymers is between 1:99 and 99:1, preferably from1:50 to 50:1.

The polyphosphoric acid used in step A) and C) is commercialpolyphosphoric acid as obtainable, for example, from Riedel-de Haen. Thepolyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) typically have acontent, calculated as P₂O₅ (by acidimetry), of at least 83%. Instead ofa solution of the monomers, it is also possible to obtain adispersion/suspension.

The mixture obtained in step A) and C) has a weight ratio ofpolyphosphoric acid to sum of all monomers of from 1:10 000 to 10 000:1,preferably from 1:1000 to 1000:1, in particular from 1:100 to 100:1.

The polymerization in steps B) and D) is carried out at a temperatureand for a time until an intrinsic viscosity of up 1.5 dl/g, preferablyfrom 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g. Typically, thetemperatures are up to 200° C., preferably up to 180° C., in particularfrom 100° C. to 180° C. The time is typically from a few minutes (5minutes) up to several hours (100 hours). The above reaction conditionsdepend upon the reactivity of the particular monomers.

The layer formation according to step F) is effected by means ofmeasures known per se (casting, spraying, knife-coating), which areknown from the prior art for polymer film production. Suitable carriersare all carriers which can be referred to as inert under the conditions.To adjust the viscosity, the solution can optionally be admixed withphosphoric acid (conc. phosphoric acid, 85%). This allows the viscosityto be adjusted to the desired value and the formation of the membrane tobe facilitated.

The layer obtained according to step F) has a thickness between 20 and4000 μm, preferably between 30 and 3500 μm, in particular between 50 and3000 μm.

The polymerization of the polyazole block polymer in step G) is carriedout at a temperature and for a time until the intrinsic viscosity ismore than 1.5 dl/g, preferably more than 1.8 dl/g, in particular morethan 1.9 dl/g. Typically, the temperatures are up to 350° C., preferablyup to 280° C. The time is typically from a few minutes (min. 1 minute)up to several hours (10 hours). The above reaction conditions dependupon the reactivity of the particular polymers, and also upon the layerthickness.

The polyazole-based block polymer formed in step G) contains repeatazole units of the general formula (I) and/or (II) and/or (III) and/or(IV) and/or (V) and/or (VI) and/or (VII) and/or (VIII) and/or (IX)and/or (X) and/or (XI) and/or (XII) and/or (XIII) and/or (XIV) and/or(XV) and/or (XVI) and/or (XVI) and/or (XVII) and/or (XVIII) and/or (XIX)and/or (XX) and/or (XXI) and/or (XXII)

in which

-   Ar are the same or different and are each a tetravalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar¹ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar² are the same or different and are each a di- or trivalent    aromatic or heteroaromatic group which may be mono- or polycyclic,-   Ar³ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁴ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁵ are the same or different and are each a tetravalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁶ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁷ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁸ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   Ar⁹ are the same or different and are each a di- or tri- or    tetravalent aromatic or heteroaromatic group which may be mono- or    polycyclic,-   Ar¹⁰ are the same or different and are each a di- or trivalent    aromatic or heteroaromatic group which may be mono- or polycyclic,-   Ar¹¹ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mono- or polycyclic,-   X are the same or different and are each oxygen, sulfur or an amino    group which bears a hydrogen atom, a group having 1-20 carbon atoms,    preferably a branched or unbranched alkyl or alkoxy group, or an    aryl group as further radical,-   R is the same or different and is hydrogen, an alkyl group or an    aromatic group, with the proviso that R in formula (XX) is not    hydrogen, and-   n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100.

Preferred aromatic or heteroaromatic groups derive from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenylsulfone, quinoline,pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotriazole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine,indolizine, quinolizine, pyridopyridine, imidazopyrimidine,pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline,phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthrolineand phenanthrene, which may optionally also be substituted.

The substitution pattern of Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ isas desired; in the case of phenylene, for example, Ar¹, Ar⁴, Ar⁶, Ar⁷,Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ may be ortho-, meta- and para-phenylene.Particularly preferred groups derive from benzene and biphenylene, whichmay optionally also be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, for example methyl, ethyl, n- or i-propyl and t-butylgroups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms, for example fluorine, aminogroups, hydroxy groups or short-chain alkyl groups, for example methylor ethyl groups.

Preference is given to polyazoles having repeat units of the formula (I)in which the X radicals are the same within one repeat unit.

The polyazoles may in principle also have different repeat units whichdiffer, for example, in their X radical. However, it preferably has onlyidentical X radicals in a repeat unit.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzooxazoles, polyoxadiazoles,polyquinoxalines, polythiadiazoles, poly(pyridines), poly(pyrimidines)and poly(tetraazapyrenes).

In a further embodiment of the present invention, the polymer containingrepeat azole units is a copolymer or a blend which contains at least twounits of the formula (I) to (XXII) which differ from one another.

The number of repeat azole units in the polymer is preferably an integergreater than or equal to 10. Particularly preferred polymers contain atleast 100 repeat azole units.

In the context of the present invention, preference is given to blockpolymers containing repeat benzimidazole units. Some examples of thehighly appropriate polymers containing repeat benzimidazole units arerepresented by the following formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

When the mixture according to step A) and C) also comprisestricarboxylic acids or tetracarboxylic acid, this achievesbranching/crosslinking of the polymer formed. This contributes toimproving the mechanical properties.

The layer obtained according to step F) is treated in the presence ofmoisture at temperatures and for a time sufficient for the layer to havesufficient strength for use in fuel cells. The treatment can be effectedto the extent that the membrane is self-supporting, so that it can beremoved from the carrier without damage.

In one variant of the process, heating the mixture from step E) totemperatures of up to 350° C., preferably up to 280° C., already bringsabout the formation of the block polymer. This can be done via themeasurement of the intrinsic viscosity. As soon as this has attained thevalues required in step G), it is possible to entirely or partlydispense with the thin-layer polymerization in step G). The time for thevariant is typically from a few minutes (20 minutes) up to several hours(40 hours). The above reaction conditions depend upon the reactivity ofthe particular monomers. This variant too forms part of the subjectmatter of the present invention.

The membrane is treated in step H) at temperatures above 0° C. and lessthan 150° C., preferably at temperatures between 10° C. and 120° C., inparticular between room temperature (20° C.) and 90° C., in the presenceof moisture or water and/or steam and/or aqueous phosphoric acid of upto 85%. The treatment is effected preferably under standard pressure,but may also be effected under the action of pressure. It is importantthat the treatment is done in the presence of sufficient moisture, as aresult of which the polyphosphoric acid present contributes to thestrengthening of the membrane by virtue of partial hydrolysis to formlow molecular weight polyphosphoric acid and/or phosphoric acid.

The partial hydrolysis of the polyphosphoric acid in step H) leads tostrengthening of the membrane and to a decrease in the layer thicknessand formation of a membrane having a thickness between 15 and 3000 μm,preferably between 20 and 2000 μm, in particular between 20 and 1500 μm,which is self-supporting. The intra- and intermolecular structurespresent in the polyphosphoric acid layer (interpenetrating networks,IPN) lead to ordered membrane formation which draws responsible for theparticular properties of the membranes formed.

The upper temperature limit of the treatment according to step H) isgenerally 150° C. In the case of extremely brief action of moisture, forexample of superheated steam, this steam may also be hotter than 150° C.The essential condition for the upper temperature limit is the durationof treatment.

The partial hydrolysis (step H) can also be effected inclimate-controlled chambers in which the hydrolysis can be controlledunder defined action of moisture. In this case, the moisture can beadjusted in a controlled manner by the temperature or saturation of thecontacting environment, for example gases such as air, nitrogen, carbondioxide or other suitable gases, or steam. The treatment time isdependent upon the parameters selected above.

The treatment time is also dependent upon the thickness of the membrane.

In general, the treatment time is between a few seconds to minutes, forexample under reaction of superheated steam, or up to whole days, forexample under air at room temperature and low relative atmosphericmoisture. The treatment time is preferably between 10 seconds and 300hours, in particular from 1 minute to 200 hours.

When the partial hydrolysis is carried out at room temperature (20° C.)with ambient air of relative atmospheric moisture content of 40-80%, thetreatment time is between 1 and 200 hours.

The membrane obtained according to step H) may be in self-supportingform, i.e. it can be removed from the carrier without damage andsubsequently optionally be further processed directly.

It is possible via the degree of hydrolysis, i.e. the time, temperatureand atmospheric moisture content, to adjust the concentration ofphosphoric acid and hence the conductivity of the inventive polymermembrane. According to the invention, the concentration of phosphoricacid is reported as mole of acid per mole of repeat unit of the polymer.In the context of the present invention, preference is given to aconcentration (mole of phosphoric acid based on a repeat unit of theformula (Ill), i.e. polybenzimidazole) of at least 20, preferably of atleast 30, in particular of at least 51. Such high degrees of doping(concentrations) are obtainable with great difficulty, if at all, bydoping polyazoles with commercially available ortho-phosphoric acid. Thepolyazole membranes described in the German patent applicationNo.10117687.2 too exhibit a high phosphoric acid content. However, theinventive block polymer membranes surpass these considerably andadditionally exhibit very good elongation at break. It is thus possibleto increase the phosphoric acid content and simultaneously to obtainimproved mechanical properties. Thus, the inventive block polymersexhibit an elongation of at least 400%, preferably of at least 500% (atfrom 1.2 to 1.8 MPa). At smaller forces of from 0.6 to 0.8 MPa, theelongations at break are more than 550%, in some cases even more than1000%.

After the treatment according to step H), the membrane can also becrosslinked on the surface by reaction of heat in the presence ofatmospheric oxygen. This curing of the membrane surface improves theproperties of the membrane additionally. The crosslinking can also beeffected by the action of IR or NIR (IR=infrared, i.e. light having awavelength of more than 700 nm; NIR=near IR, i.e. light having awavelength in the range from approx. 700 to 2000 nm or an energy in therange from approx. 0.6 to 1.75 eV). A further method is irradiation withβ-rays. The radiation dose here is between 5 and 200 kGy.

The inventive block polymer membrane has improved material propertiescompared to the doped polymer membranes known to date. In particular,they exhibit better performance in comparison with known doped polymermembranes. The reason for this is in particular improved protonconductivity. At temperatures of 160° C., this is at least 0.13 S/cm,preferably at least 0.14 S/cm, in particular at least 0.15 S/cm.

To further improve the performance properties, it is additionallypossible to add fillers, especially proton-conducting fillers, and alsoadditional acids to the membrane. The addition may be effected either instep A and/or C or after the polymerization (step B and/or D or E)

Nonlimiting examples of proton-conducting fillers are

-   Sulfates such as: CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   Phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   Polyacid such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW_(12O) ₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   Selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   Oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   Silicates such as zeolites, zeolites(NH₄+), sheet silicates,    framework silicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites-   Acids such as HCIO₄, SbF₅-   Fillers such as carbides, in particular SiC, Si₃N₄, fibers, in    particular glass fibers, glass powders and/or polymer powders,    preferably based on polyazoles.

In addition, this membrane may also contain perfluorinated sulfonic acidadditives (0.1-20% by weight, preferably 0.2-15% by weight, verypreferably 0.2-10% by weight). These additives lead to enhancement ofperformance, to an increase in the oxygen solubility and oxygendiffusion close to the cathode and to a reduction in the adsorption ofphosphoric acid and phosphate on platinum. (Electrolyte additives forphosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.; Olsen, C.; Berg,R. W.; Bjerrum, N. J. Chem. Dep. A, Tech. Univ. Denmark, Lyngby, Den. JtElectrochem. Soc. (1993), 140(4), 896-902 and Perfluorosulfonimide as anadditive in phosphoric acid fuel cell. Razaq, M.; Razaq, A.; Yeager, E.;DesMarteau, Darryl D.; Singh, S. Case Cent. Electrochem. Sci., CaseWest. Reserve Univ., Cleveland, Ohio, USA. J. Electrochem. Soc. (1989),136(2), 385-90.)

Nonlimiting examples of persulfonated additives are:trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,sodium trifluoromethanesulfonate, lithium trifluoromethanesulfonate,ammonium trifluoromethanesulfonate, potassium perfluorohexanesulfonate,sodium perfluorohexanesulfonate, lithium perfluorohexanesulfonate,ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,potassium nonafluorobutanesulfonate, sodium nonafluorobutane sulfonate,lithium nonafluorobutanesulfonate, ammonium nonafluorobutanesulfonate,cesium nonafluorobutanesulfonate, triethylammoniumperfluorohexanesulfonate, perfluorosulfonimides and Nafion.

In addition, the membrane may also comprise as additives which scavenge(primary antioxidants) or destroy (secondary antioxidants) the peroxideradicals generated in oxygen reduction in the course of operation andthus, as described in JP2001118591 A2, improve lifetime and stability ofthe membrane and membrane-electrode unit.

The way in which such additives function and their molecular structuresare described in F. Gugumus in Plastics Additives, Hanser Verlag, 1990;N.S. Allen, M. Edge Fundamentals of Polymer Degradation and Stability,Elsevier, 1992; or H. Zweifel, Stabilization of Polymeric Materials,Springer, 1998.

Nonlimiting examples of such additives are: bis(trifluoromethyl)nitroxide, 2,2-diphenyl-1-picrinylhydrazyl, phenols, alkylphenols,sterically hindered alkylphenols, for example Irganox, aromatic amines,sterically hindered amines, for example Chimassorb; sterically hinderedhydroxylamines, sterically hindered alkylamines, sterically hinderedhydroxylamines, sterically hindered hydroxylamine ethers, phosphites,for example Irgafos, nitrosobenzene, methyl-2-nitrosopropane,benzophenone, benzaldehyde tert-butyl nitron, cysteamine, melanines,lead oxides, manganese oxides, nickel oxides, cobalt oxides.

Possible fields of use of the inventive doped polymer membranes includeuse in fuel cells, in electrolysis, in capacitors and in batterysystems. Owing to their property profile, the doped polymer membranesare preferably used in fuel cells.

The present invention also relates to a membrane-electrode unit whichhas at least one inventive polymer membrane. For further informationabout membrane-electrode units, reference is made to the technicalliterature, especially to the patents US-A-4,191,618, US-A-4,212,714 andUS-A-4,333,805. The disclosure present in the aforementioned references[US-A-4,191,618, US-A-4,212,714 and US-A-4,333,805] with regard to theconstruction and the production of membrane-electrode units, and alsothe electrodes, gas diffusion layers and catalysts to be selected, alsoforms part of the description.

In one variant of the present invention, the membrane can also formdirectly on the electrode instead of on a carrier. This allows thetreatment according to step H) to be shortened appropriately, since themembrane no longer has to be self-supporting. Such a membrane also formspart of the subject matter of the present invention.

The present invention further provides an electrode which having aproton-conducting polymer coating based on polyazoles, obtainable by aprocess comprising the steps of

-   A) mixing one or more aromatic tetraamino compounds having a high    phosphoric acid affinity or low phosphoric acid affinity with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer which have a high    phosphoric acid affinity or low phosphoric acid affinity, or mixing    one or more aromatic and/or heteroaromatic diaminocarboxylic acids    having a high phosphoric acid affinity in polyphosphoric acid to    form a solution and/or dispersion-   B) heating the mixture from step A), preferably under inert gas, and    polymerizing until an intrinsic viscosity of up to 1.5 dl/g,    preferably from 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g,    is obtained to form a polymer whose phosphoric acid affinity is    greater than the phosphoric acid affinity of the polymer formed in    step D),-   C) mixing one or more aromatic tetraamino compounds having a high    phosphoric acid affinity or low phosphoric acid affinity with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer which have a high    phosphoric acid affinity or low phosphoric acid affinity, in    polyphosphoric acid to form a solution and/or dispersion-   D) heating the mixture from step C), preferably under inert gas, and    polymerizing until an intrinsic viscosity of up to 1.5 dl/g,    preferably from 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g,    is obtained to form a polymer whose phosphoric acid affinity is less    than the phosphoric acid affinity of the polymer formed in step B),-   E) combining the polymer from step B) and the polymer from step D),    the phosphoric acid affinity of the polymer from step B) being    greater than the phosphoric acid affinity of the polymer from step    D),-   F) applying a layer using the mixture according to step E) on an    electrode,-   G) heating the sheetlike structure/layer obtainable according to    step F), preferably under inert gas, until an intrinsic viscosity of    more than 1.5 dl/g, preferably of more than 1.8 dl/g, in particular    of more than 2.0 dl/g, is attained to form a polyazole block    polymer,-   H) treating the membrane formed in step G).

The variants and preferred embodiments described above are also validfor this subject matter, so that there is no need to repeat them at thispoint.

After step H), the coating has a thickness between 2 and 3000 μm,preferably between 3 and 2000 μm, in particular between 5 and 1500 μm.

An electrode coated in this way can be installed in a membrane-electrodeunit which optionally has at least one inventive block polymer membrane.

General Test Methods

Test Method for IEC

The conductivity of the membrane depends greatly upon the content ofacid groups expressed by the so-called ion exchange capacity (IEC). Tomeasure the ion exchange capacity, a sample with a diameter of 3 cm isstamped out and introduced into a beaker filled with 100 ml of water.The released acid is titrated with 0.1 M NaOH. Subsequently, the sampleis withdrawn, excess water is dabbed off and the sample is dried at 160°C. over 4 h. The dry weight, m₀, is then determined gravimetrically witha precision of 0.1 mg. The ion exchange capacity is then calculated fromthe consumption of 0.1 M NaOH up to the first titration end point, V₁ inml, and the dry weight, m₀ in mg, by the following formula:IEC=V₁*300/m₁Test Method for Specific Conductivity

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in potentiostatic mode and using platinumelectrodes (wire, diameter 0.25 mm). The distance between thecurrent-collecting electrodes is 2 cm. The resulting spectrum isevaluated with a simple model consisting of a parallel arrangement of anohmic resistance and a capacitor. The sample cross section of thephosphoric acid-doped membrane is measured immediately before the samplemounting. To measure the temperature dependence, the test cell isbrought to the desired temperature in an oven and controlled by means ofa Pt-100 thermoelement positioned in the immediate vicinity of thesample. On attainment of the temperature, the sample is kept at thistemperature for 10 minutes before the start of the measurement.

1. A proton-conducting polymer membrane based on polyazoles, obtainableby a process comprising the steps of A) mixing one or more aromatictetraamino compounds having a high phosphoric acid affinity or lowphosphoric acid affinity with one or more aromatic carboxylic acids oresters thereof which contain at least two acid groups per carboxylicacid monomer which have a high phosphoric acid affinity or lowphosphoric acid affinity, or mixing one or more aromatic and/orheteroaromatic diaminocarboxylic acids having a high phosphoric acidaffinity in polyphosphoric acid to form a solution and/or dispersion B)heating the mixture from step A), preferably under inert gas, andpolymerizing until an intrinsic viscosity of up to 1.5 dl/g, preferablyfrom 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g, is obtained toform a polymer whose phosphoric acid affinity is greater than thephosphoric acid affinity of the polymer formed in step D), C) mixing oneor more aromatic tetraamino compounds having a high phosphoric acidaffinity or low phosphoric acid affinity with one or more aromaticcarboxylic acids or esters thereof which contain at least two acidgroups per carboxylic acid monomer which have a high phosphoric acidaffinity or low phosphoric acid affinity, in polyphosphoric acid to forma solution and/or dispersion D) heating the mixture from step C),preferably under inert gas, and polymerizing until an intrinsicviscosity of up to 1.5 dl/g, preferably from 0.3 to 1.0 dl/g, inparticular from 0.5 to 0.8 dl/g, is obtained to form a polymer whosephosphoric acid affinity is less than the phosphoric acid affinity ofthe polymer formed in step B), E) combining the polymer from step B) andthe polymer from step D), the phosphoric acid affinity of the polymerfrom step B) being greater than the phosphoric acid affinity of thepolymer from step D), F) applying a layer using the mixture according tostep E) on a carrier or on an electrode, G) heating the sheetlikestructure/layer obtainable according to step F), preferably under inertgas, until an intrinsic viscosity of more than 1.5 dl/g, preferably ofmore than 1.8 dl/g, in particular of more than 1.9 dl/g, is attained toform a polyazole block polymer, H) treating the membrane formed in stepG) (until it is self-supporting).
 2. The membrane as claimed in claim 1,characterized in that the aromatic tetraamino compounds having a highphosphoric acid affinity used are 2,3,5,6-tetraaminopyridine,3,3′,4,4′-tetraaminodiphenylsulfone, 3,3′,4,4′-tetraaminodiphenyl etherand salts thereof, especially the mono-, di-, tri- andtetrahydrochloride derivatives thereof.
 3. The membrane as claimed inclaim 1, characterized in that the aromatic tetraamino compounds havinga low phosphoric acid affinity used are 3,3′,4,4′-tetraaminobiphenyl,1,2,4,5-tetraaminobenzene, 3,3′,4,4′-tetraaminobenzophenone,3,3′,4,4′-tetraaminodiphenylmethane and3,3′,4,4′-tetraaminodiphenyldimethylmethane and salts thereof,especially the mono-, di-, tri- and tetrahydrochloride derivativesthereof.
 4. The membrane as claimed in claim 1, characterized in thatthe aromatic carboxylic acids having a high phosphoric acid affinityused are pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid,2-hydroxyterephthalic acid, 5-aminoisophthalic acid,5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic acid,2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,8-dihydroxynaphthalene-3,6-dicarboxylic acid anddiphenylsulfone-4,4′-dicarboxylic acid.
 5. The membrane as claimed inclaim 1, characterized in that the aromatic carboxylic acids having alow phosphoric acid affinity used are isophthalic acid, terephthalicacid, phthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalic acid,2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, diphenyl ether 4,4′-dicarboxylic acid,benzophenone-4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid and 4-carboxycinnamic acid.
 6. The membrane as claimed in claim 1,characterized in that the diaminocarboxylic acids having a highphosphoric acid affinity used are diaminobenzoic acid and the mono anddihydrochloride derivatives thereof, and also 1,2-diamino-3′-carboxyacid 4,4′-diphenyl ether.
 7. The membrane as claimed in claim 1,characterized in that the aromatic carboxylic acids used aretricarboxylic acids, tetracarboxylic acids or the C1-C20-alkyl esters orC5-C12-aryl esters thereof or the acid anhydrides thereof or the acidchlorides thereof, preferably 1,3,5-benzenetricarboxylic acid (trimesicacid); 1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid, 3,5,3′- biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid and/or 2,4,6-pyridinetricarboxylicacid.
 8. The membrane as claimed in claim 1, characterized in that thearomatic carboxylic acids used are tetracarboxylic acids, theC1-C20-alkyl esters or C5-C12-aryl esters thereof or the acid anhydridesthereof or the acid chlorides thereof, preferablybenzene-1,2,4,5-tetracarboxylic acids;naphthalene-1,4,5,8-tetracarboxylic acids,3,5,3′,5′-biphenyltetracarboxylic acid; benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.
 9. The membrane as claimed inclaim 4, characterized in that the content of tricarboxylic acid ortetracarboxylic acids (based on dicarboxylic acid used) is between 0 and30 mol%, preferably 0.1 and 20 mol%, in particular 0.5 and 10 mol%. 10.The membrane as claimed in claim 1, characterized in that theheteroaromatic carboxylic acids used are heteroaromatic dicarboxylicacids and tricarboxylic acids and tetracarboxylic acids which contain atleast one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic,preferably pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylicacid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,and also the C1-C20-alkyl esters or C5-C12-aryl esters thereof, or theacid anhydrides thereof or the acid chlorides thereof.
 11. The membraneas claimed in claim 1, characterized in that a polyphosphoric acidhaving a content, calculated as P₂O₅ (by acidimetry), of at least 83% isobtained in step A) and C).
 12. The membrane as claimed in claim 1,characterized in that a solution or a dispersion/suspension is obtainedin step A) and C).
 13. The membrane as claimed in claim 1, characterizedin that block polymers based on polyazole and comprising repeat azoleunits of the general formula (I) and/or (II) and/or (III) and/or (IV)and/or (V) and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X)and/or (XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or(XVI) and/or (XVI) and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX)and/or (XXI) and/or (XXII)

in which Ar are the same or different and are each a tetravalentaromatic or heteroaromatic group which may be mono- or polycyclic, Ar₁are the same or different and are each a divalent aromatic orheteroaromatic group which may be mono- or polycyclic, Ar² are the sameor different and are each a di- or trivalent aromatic or heteroaromaticgroup which may be mono- or polycyclic, Ar³ are the same or differentand are each a trivalent aromatic or heteroaromatic group which may bemono- or polycyclic, Ar⁴ are the same or different and are each atrivalent aromatic or heteroaromatic group which may be mono- orpolycyclic, Ar⁵ are the same or different and are each a tetravalentaromatic or heteroaromatic group which may be mono- or polycyclic, Ar6are the same or different and are each a divalent aromatic orheteroaromatic group which may be mono- or polycyclic, Ar⁷ are the sameor different and are each a divalent aromatic or heteroaromatic groupwhich may be mono- or polycyclic, Ar⁸ are the same or different and areeach a trivalent aromatic or heteroaromatic group which may be mono- orpolycyclic, Ar⁹ are the same or different and are each a di- or tri- ortetravalent aromatic or heteroaromatic group which may be mono- orpolycyclic, Ar¹⁰ are the same or different and are each a di- ortrivalent aromatic or heteroaromatic group which may be mono- orpolycyclic, Ar¹¹ are the same or different and are each a divalentaromatic or heteroaromatic group which may be mono- or polycyclic, X arethe same or different and are each oxygen, sulfur or an amino groupwhich bears a hydrogen atom, a group having 1-20 carbon atoms,preferably a branched or unbranched alkyl or alkoxy group, or an arylgroup as further radical, R is the same or different and is hydrogen, analkyl group or an aromatic group, with the proviso that R in formula(XX) is not hydrogen, and n, m are each an integer greater than or equalto 10, preferably greater than or equal to 100, are formed in step G).14. The membrane as claimed in claim 1, characterized in that a blockpolymer containing repeat segments selected from the group ofpolybenzimidazole, poly(pyridines), poly(pyrimidines), polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles and poly(tetraazapyrenes) is formed in step G).
 15. Themembrane as claimed in claim 1, characterized in that a block polymercontaining repeat benzimidazole units of the formula

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100, is formed in step G).
 16. Themembrane as claimed in claim 1, characterized in that the membraneobtained in step H) is treated in the presence of moisture attemperatures and for a period until the membrane is self-supporting andcan be removed from the carrier without damage.
 17. The membrane asclaimed in claim 1, characterized in that the membrane is treated instep H) at temperatures above 0° C. and 150° C., preferably attemperatures between 10° C. and 120° C., in particular between roomtemperature (20° C.) and 90° C., in the presence of moisture or waterand/or steam.
 18. The membrane as claimed in claim 1, characterized inthat the treatment of the membrane in step H) is between 10 seconds and300 hours, preferably from 1 minute to 200 hours.
 19. The membrane asclaimed in claim 1, characterized in that the carrier selected in stepF) is an electrode and the treatment in step H) is such that themembrane formed is no longer self-supporting.
 20. The membrane asclaimed in claim 1, characterized in that a layer having a thickness of20 and 4000 μm, preferably between 30 and 3500 μm, in particular between50 and 3000 μm is obtained in step F).
 21. The membrane as claimed inclaim 1, characterized in that the membrane formed by step H) has athickness between 15 and 3000 μm, preferably between 20 and 2000 μm, inparticular between 20 and 1500 μm.
 22. An electrode which having aproton-conducting polymer coating based on polyazoles, obtainable by aprocess comprising the steps of A) mixing one or more aromatictetraamino compounds having a high phosphoric acid affinity or lowphosphoric acid affinity with one or more aromatic carboxylic acids oresters thereof which contain at least two acid groups per carboxylicacid monomer which have a high phosphoric acid affinity or lowphosphoric acid affinity, or mixing one or more aromatic and/orheteroaromatic diaminocarboxylic acids having a high phosphoric acidaffinity in polyphosphoric acid to form a solution and/or dispersion B)heating the mixture from step A), preferably under inert gas, andpolymerizing until an intrinsic viscosity of up to 1.5 dl/g, preferablyfrom 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g, is obtained toform a polymer whose phosphoric acid affinity is greater than thephosphoric acid affinity of the polymer formed in step D), C) mixing oneor more aromatic tetraamino compounds having a high phosphoric acidaffinity or low phosphoric acid affinity with one or more aromaticcarboxylic acids or esters thereof which contain at least two acidgroups per carboxylic acid monomer which have a high phosphoric acidaffinity or low phosphoric acid affinity, in polyphosphoric acid to forma solution and/or dispersion D) heating the mixture from step C),preferably under inert gas, and polymerizing until an intrinsicviscosity of up to 1.5 dl/g, preferably from 0.3 to 1.0 dl/g, inparticular from 0.5 to 0.8 dl/g, is obtained to form a polymer whosephosphoric acid affinity is less than the phosphoric acid affinity ofthe polymer formed in step B), E) combining the polymer from step B) andthe polymer from step D), the phosphoric acid affinity of the polymerfrom step B) being greater than the phosphoric acid affinity of thepolymer from step D), F) applying a layer using the mixture according tostep E) on an electrode, G) heating the sheetlike structure/layerobtainable according to step F), preferably under inert gas, until anintrinsic viscosity of more than 1.5 dl/g, preferably of more than 1.8dl/g, in particular of more than 2.0 dl/g, is attained to form apolyazole block polymer, H) treating the membrane formed in step G). 23.The electrode as claimed in claim 22, the coating having a thicknessbetween 2 and 3000 μm, preferably between 3 and 2000 μm, in particularbetween 5 and 1500 μm.
 24. A membrane-electrode unit comprising at leastone electrode and at least one membrane as claimed in claim
 1. 25. Amembrane-electrode unit comprising at least one electrode as claimed inclaim 22 and at least one membrane based on polyazoles, obtainable by aprocess comprising the steps of A) mixing one or more aromatictetraamino compounds having a high phosphoric acid affinity or lowphosphoric acid affinity with one or more aromatic carboxylic acids oresters thereof which contain at least two acid groups per carboxylicacid monomer which have a high phosphoric acid affinity or lowphosphoric acid affinity, or mixing one or more aromatic and/orheteroaromatic diaminocarboxylic acids having a high phosphoric acidaffinity in polyphosphoric acid to form a solution and/or dispersion B)heating the mixture from step A), preferably under inert gas, andpolymerizing until an intrinsic viscosity of up to 1.5 dl/g, preferablyfrom 0.3 to 1.0 dl/g, in particular from 0.5 to 0.8 dl/g, is obtained toform a polymer whose phosphoric acid affinity is greater than thephosphoric acid affinity of the polymer formed in step D), C) mixing oneor more aromatic tetraamino compounds having a high phosphoric acidaffinity or low phosphoric acid affinity with one or more aromaticcarboxylic acids or esters thereof which contain at least two acidgroups per carboxylic acid monomer which have a high phosphoric acidaffinity or low phosphoric acid affinity, in polyphosphoric acid to forma solution and/or dispersion D) heating the mixture from step C),preferably under inert gas, and polymerizing until an intrinsicviscosity of up to 1.5 dl/g, preferably from 0.3 to 1.0 dl/g, inparticular from 0.5 to 0.8 dl/g, is obtained to form a polymer whosephosphoric acid affinity is less than the phosphoric acid affinity ofthe polymer formed in step B), E) combining the polymer from step B) andthe polymer from step D), the phosphoric acid affinity of the polymerfrom step B) being greater than the phosphoric acid affinity of thepolymer from step D), F) applying a layer using the mixture according tostep E) on a carrier or on an electrode, G) heating the sheetlikestructure/layer obtainable according to step F), preferably under inertgas, until an intrinsic viscosity of more than 1.5 dl/g, preferably ofmore than 1.8 dl/g, in particular of more than 1.9 dl/g, is attained toform a polyazole block polymer, H) treating the membrane formed in stepG) (until it is self-supporting).
 26. A fuel cell comprising one or moremembrane-electrode units as claimed in claim
 24. 27. A fuel cellcomprising one or more membrane-electrode units as claimed in claim 25.